This invention relates to a dimming method and device for fluorescent lamps to be used in a backlighting system for liquid crystal visual displays.
Liquid crystal screens, more particularly those used for color visual display on instrument panels in airplanes and helicopters, are equipped with backlighting systems which provide a high level of brightness making them comfortably visible even with strong ambient light. This brightness must be variable allowing it to be adapted to the various intensities of ambient light, and this brightness must also be adaptable to day-night ambient variations. Such variations imply a light dimming ratio of 1000:1, which for fluorescent lamps corresponds to a brightness intensity of a few Cd/m.sup.2 for minimum brightness and approximately 15,000 Cd/m.sup.2 for maximum brightness.
It is to be noted that the light source uses fluorescent lamps due to their high energy efficiency and to their colorimetry which is well-adapted to liquid crystal screens.
To obtain an optimal brightness level with these lamps, the power supply voltage which is applied between their two electrodes is a high alternating voltage, generally between 300 and 500 volts, at a frequency of several tens of kilohertz.
As is well-known in the art, it is possible to vary the brightness of a fluorescent lamp by varying the amplitude of the power voltage and consequently, the current traversing the lamp. This method is only capable of producing a brightness dimming ratio of 10:1, which is insufficient for the above-mentioned application. Moreover, the fact that the triggering voltage of a fluorescent lamp is dependent on the temperature, more precisely, that this voltage increases as temperature falls, implies that this brightness control method does not allow operation over a wide temperature range, especially when the temperature is below 0.degree. C.
It is generally known that the range of brightness levels can be improved by modulating the frequency of the alternating supply voltage and, more precisely, by using, for example, square waves of frequency varying from tens of hertz to tens of kilohertz. In this case, however, to satisfy the aforementioned conditions of operation, it is necessary to work with frequencies of less than 15 kilohertz in order to produce low brightness levels and at these frequencies sound vibrations may result. Finally, at a very low brightness level there appears a flickering due to stroboscopic effect between the intermittent ignition of lamps and the refreshing of the image of which the frequency is between 50 and 60 hertz. This results in a bright horizontal bar on the screen which is absolutely unacceptable for pilot control displays.
As is also well-known in the art, the brightness of a fluorescent lamp can be varied by applying a square wave voltage with an adjustable duty cycle width. However, there exist problems with respect to stroboscopic effect in this method too.